1. Field of the Invention
The present invention relates to a method for producing a single crystal or polycrystal of terbium aluminate, which is useful as a luminescent material such as a laser material or a phosphor.
2. Discussion of Background
Heretofore, a luminescent material has been prepared mainly by incorporating a very small amount of an optically active element to an optically inert matrix. As luminescent materials containing an optically active element as a matrix-constituting element, pentaphosphates (e.g. NbP.sub.5 O.sub.14) are known which were invented in 1970s. However, substances of this type are produced by polymerization of phosphoric acid and thus have many drawbacks, for example, such that the furnace for their synthesis is badly damaged.
On the other hand, Tb.sub.3 Al.sub.5 O.sub.12 and TbAl.sub.3 B.sub.4 O.sub.12 prepared by a flux method employing lead fluoride as the main flux, have been reported as compounds containing terbium as a matrix-constituting element. Each compound presents a brown or black color. However, such an emission is known to be completely extinct at a liquid nitrogen temperature (i.e. an absolute temperature of 77.degree.), and there is no report that the compound exhibits an emission at room temperature. This is attributable to the fact that by a flux method, it is difficult to control the valence of terbium ions. Namely, terbium is supplied in the form of Tb.sub.4 O.sub.7 for industrial purposes, and the starting material for synthesis contains trivalent terbium and tetravalent terbium in a ratio of 1:1. Accordingly, by the flux method, it has been difficult to remove tetravalent terbium which is considered to absorb the emission energy, or it is impossible to reduce it to trivalent terbium.